1. Field of the Invention
The present invention relates to a method for producing anthracene from phenanthrene and to methods providing improved yields of intermediate products.
2. Brief Description of the Background of the Invention Including Prior Art
Phenanthrene and anthracene are generally obtained from bituminous coal tar containing about five weight percent of phenanthrene and from about 1.2 to 1.4 weight percent of anthracene. However, anthracene is the more valuable product since it serves in increasing amount in the production of anthraquinone coloring agents. Therefor, there has been no lack in attempts to transform phenanthrene into anthracene. The long known process of hydrogenating phenanthrene to sym.-octahydrophenanthrene, isomerizing the sym.-octahydrophenanthrene to sym.-octahydroanthracene and dehydrogenating the sym.-octahydroanthracene to anthracene has been subject of a number of proposals trying to improve the yield and to simplify technically the reaction steps.
It is already known to perform the hydrogenation of phenanthrene such as to yield to a large extent sym.-octahydrophenanthrene [Journal of the American Chemical Society 59 (1937) 135 and 60 (1938) 1501-1505]. It is disclosed that in the presence of a nickel on a carrier catalyst about 9 percent asym.-octahydrophenanthrene and about 91 percent sym.-octahydrophenanthrene are obtained from phenanthrene dissolved in diethylcyclohexane at a temperature of 191.degree. C. to 198.degree. C. and under a hydrogen pressure of 56 bar after 3.5 hours and the sym.-octahydroanthracene is separated from the reaction mixture by distillation [German Offenlegungsschrift DE-OS No. 16 18 597]. The easy formation of undesired perhydrophenanthrene (tetradecahydrophenanthrene) is in fact suppressed by the employment of a solvent, however the cost of the process is increased.
It is also known to isomerize sym.-octahydrophenanthrene to sym.-octahydroanthracene in the presence of aluminum chloride at 70.degree. C. to 80.degree. C. The resulting mixture contains about 12 percent of higher condensed by-products and the remaining 88 percent contain to about half in each case of the two isomers [Berichte 57 (1924) 1990-2024]. It is furthermore known to perform the isomerisation with 2.5 to 5 weight percent aluminum chloride based on the sym.-octahydrophenanthrene at a temperature of 5.degree. to 45.degree. C. and preferably 20.degree. to 35.degree. C. during a reaction time of from about 10 to 24 hours.
Following, the product is taken up with toluene and is processed as usual. About 93 to 96 weight percent of the starting material amount are recovered, which are composed as follows:
87-90 weight percent mixture of the isomers of sym.-octahydroanthracene and sym.-octahydrophenanthrene as well as 10-13 weight percent of higher condensed by-products. The contents of the isomer mixture in sym.-octahydroanthracene amounts to 81-96 weight percent [German patent disclosure DE-AS 857 042].
In view of the whole process, the isomerization of the sym.-octahydrophenathrene is of particular importance, since a high degree of isomerization and a low formation of by-products are decisive for the success of the process.